This invention relates to a bicycle rear derailleur, and more particularly to improvements in the rear derailleur for increasing its gear capacity (ability of a rear derailleur to compensate a sagging in an endless drive chain to keep it in tension).
As is well known, a multi-speed type bicycle is equipped with a multiple freewheel, which usually has three to eight different sized sprockets, and a rear derailleur for selectively shifting the drive chain from one sprocket to another sprocket of the multiple freewheel in order to effect a desired speed change. It is also well known that some of the multi-speed type bicycle is equipped with a multiple chainwheel (crank gear) and a front derailleur in addition to the multiple freewheel and the rear derailleur. The multiple chainwheel usually has two to three different sized sprockets, and the front derailleur is used for selectively shifting the drive chain from one sprocket to another sprocket of the chainwheel in order to effect a desired speed change.
In the multi-speed type bicycle equipped with both of the multiple freewheel and the multiple chainwheel, the number of available speed change gear positions is given by the number of the freewheel sprockets multiplied by the number of the chainwheel sprockets. For instance, in such a particular bicycle that is equipped with a multiple freewheel having six different sized sprockets and a multiple chainwheel having two different sized sprockets, the speed change of the bicycle is available at twelve (6.times.2) stages.
Thus, it is apparent that the bicycle equipped with both of the multiple freewheel and the multiple chainwheel generally has a wide range of speed change.
On the other hand, it is well known that a length of the endless chain for driving a bicycle is constant in whatever speed gear position it may be, while a length of a running course of the chain varies in accordance with a selected speed gear position. Naturally, when the chain is shifted from a larger sprocket to a smaller sprocket, the length of the running course of the chain is reduced, resulting in that the chain has a certain excess in its length, producing a certain number of excess link plates of the chain. Thus, the chain loses its tension to produce an undesirable sagging, which will cause disengagement of the chain from the sprockets. Therefore, it is necessary that the rear derailleur is provided with a tension mechanism so arranged as to compensate the sagging in the chain and always arranged as to compensate keep the chain in proper tension. The maximum sagging in the chain is produced when the chain is in engagement with the smallest freewheel sprocket and the smallest chainwheel sprocket, where the excess chain link plates, which causes the sagging and therefore should be absorbed for giving a tension, is greatest in number.
The ability of the rear derailleur to compensate the sagging of the drive chain to keep it in proper tension is called "gear capacity of a rear derailleur" (hereinafter referred to as "gear capacity"). A degree of the gear capacity is defined as the maximum number of the excess chain link plates which the rear derailleur can compensate to keep the chain in tension, and such number corresponds to a difference between the number of the teeth of the largest sprocket and that of the teeth of the smallest sprocket.
In the bicycle equipped with a combination of the multiple freewheel and the multiple chainwheel, the maximum number of the excess chain link plates corresponds to the sum of a difference between the numbers of the teeth of the largest and the smallest sprockets of the multiple freewheel and that between the numbers of the teeth of the largest and the smallest sprockets of the multiple chainwheel.
For example, in a multi-speed type bicycle equipped with such a multiple freewheel as having a largest sprocket with 34 teeth and a smallest sprocket with 13 teeth and such a multiple chainwheel as having a largest sprocket with 52 teeth and a smallest sprocket with 28 teeth, a gear capacity required in the rear derailleur to be used with this particular multi-speed bicycle is 45T as a result of the following calculation: EQU (34-13)+(52-28)=45T
wherein "T" is usually added to indicate that the value is derived from the number of the teeth corresponding to the number of excess chain link plates to be absorbed to keep the chain in proper tension.
The rear derailleur having a large gear capacity is advantageous when utilized for a so-called mountain bike (a bicycle specially designed for off-road cycling--See FIG. 1 of the accompanying drawing). In the mountain bike, a diameter of the largest sprocket of the multiple freewheel must be large enough while a diameter of the smallest sprocket of the chainwheel must be small enough, so that a cyclist can make an easy ascent of a steep slope of a hillside or the like steadily and slowly. On the other hand, a diameter of the smallest sprocket of the freewheel must be small enough while a diameter of the largest sprocket of the chainwheel must be large enough, so that the cyclist can enjoy cycling on a plain or downward road or field at a high speed. Thus, it is necessary that the rear derailleur for use with a mountain bike should have a large gear capacity.
The large gear capacity can be easily obtained by simply extending a length of a tension pulley support arm so as to increase a distance between a guide pulley and a tension pulley of the rear derailleur. However, this simple and easy way has never been employed in the manufacture of the mountain bikes because an increase in the length of the tension pulley support arm causes increase in danger of collision of the tension pulley against rocks or other obstacles on the off-road ground. Further, the downwardly extended tension pulley support arm gives an unattractive appearance to a bicycle.
Thus, the maximum gear capacity of various rear derailleurs now available in the markets is 37T at most. However, such a low degree of the gear capacity is not satisfactory to the mountain bike.
It is, therefore, an object of the present invention to provide an improved rear derailleur which permits a considerable increase in a gear capacity without a downward extension of a tension pulley support arm.
Another object of the invention is to provide an improved rear derailleur which is particularly advantageous when utilized with a mountain bike.
A further object of the invention is to provide an improved mechanism for a large gear capacity which is applicable to various types of existing rear derailleurs.
For the achievement of the above objects, the bicycle rear derailleur according to the invention comprises a tensioner frame for rotatably supporting a guide pulley and a tension pulley, a shifter for shifting the tensioner frame in the directions substantially parallel with respect to an axis of a rear wheel hub, the tensioner frame including a tension pulley support arm which is resiliently urged to pivotally move rearwardly of the bicycle in order to give an adjusted tension to an endless drive chain in cooperation with the tension pulley rotatably supported at a free end of the tension pulley support arm, and a compensating mechanism for compensating a sagging in the endless drive chain which is arranged so as to stretch out the endless drive chain rearwardly of the bicycle so that a distance between a geometrical vertical line passing through a center of the tension pulley and a vertical tangential line at a rearmost point of a rearwardly stretched portion of the endless drive chain is larger than a radius of a guide pulley when the tension pulley support arm takes its rearmost position. In one embodiment of the invention, the aforesaid distance is enlarged by provision of an auxiliary tension pulley. In another embodiment of the invention, the distance in question is enlarged by an enlarged tension pulley whose diameter is larger than that of the guide pulley.
As a result of provision of such specific tension mechanism, an entire length of the running course of the chain which is in engagement with the smallest freewheel sprocket and the smallest chainwheel sprocket is extended thereby to provide a large gear capacity.